It is well recognized by the petroleum refining industry that the higher boiling portions of crude oils and particularly that portion of the crude oil charged to vacuum distillation comprise a major portion of the contaminating metal compounds as well as carbon producing components referred to as Conradson carbon. More particularly, the metal bearing compounds and Conradson carbon components tend to be concentrated in the resid portion of crude oils boiling above about 552.degree. C. (1025.degree. F.). The metal contaminants in the crude oil feed and residual portions thereof comprise iron, nickel, vanadium and copper normally in the salt, oxide or metal form, depending on source. It is known that such metal contaminants rapidly deactivate cracking catalysts. Thus, until recently, relatively clean feeds reduced in metal contaminants and Conradson carbon producing components have been processed over the crystalline zeolite containing cracking catalyst. However, with the advent of crude shortage and high price, it has become necessary to process more of the crude barrel comprising the high boiling residual portion thereof known as resid or as a reduced crude. The effect of processing such higher levels of contamination metals and Conradson carbon producing component feed materials over active zeolite containing cracking catalyst normally employed for gas oil cracking is to increase the yield of dry gas and coke, rapidly destroy the active and expensive zeolite component and reduce yields of desired products such as gasoline and distillates.
An object of the present invention therefore, is to provide a process and method of operation which will provide a demetallized and decarbonized gas oil containing feed from crude oil and residual portions of crude oils in substantial higher yield and more suitable for processing over a downstream more selective and active crystalline zeolite containing fluid cracking catalyst. Another object is to obtain a heavy gas oil feed from crude oil more suitable for a fluid zeolite catalyst cracking process which will substantially reduce metal contaminant and Conradson carbon deposition on the zeolite cracking catalyst, whereby, dry gas and coke yield may be kept to a desired relatively low level. Another object of this invention is to provide an improved method for demetallizing and decarbonization a residual portion of crude oil to provide a more clean heavy oil feed for catalytic cracking upgrading over a gasoline selective crystalline zeolite containing fluid cracking catalyst. A still further object of this invention is to provide an improved and selective hydrovisbreaking operation providing substantial metals and Conradson carbon deposition and end boiling point reduction of the heavy oil charged thereto.
Visbreaking has been identified as a process where reduced crudes are pyrolized without substantial coke production. The process was initially developed to lower viscosities and pour points of residual oil feed stocks. Gasoline boiling range material and gas oils are produced with the amount of each, depending upon the type and severity of the operation employed. The products from any pyrolysis operation depends substantially upon the temperature, contact time and method of implementing the process. An object of the present invention is to provide a versatile but selective hydrovisbreaking operation for use in upgrading different or a combination of crude oil source materials to provide a more suitable feed for fluid zeolite catalytic cracking to high yields of desired product such as gasoline, gasoline precursors and light fuel oils. Other objects and advantages will be more apparent from the following discussion.